Generally, conventional solid propellant rocket motors have their fuel and oxidizers premixed. These rocket motors are ignited often by explosive squibs. These solid propellants rocket motors may only be ignited once and cannot be extinguished and restarted for later use during the launch process. Another common rocket motor is an all liquid propellant motors such as liquid hydrogen and liquid oxygen motors. These rocket motors may be extinguished and restarted, but need to utilize an external ignition source. These sources add complexity to the motor and can be bulky, thus making the rocket motor inefficient.
Hybrid rocket motors utilize solid fuel and either gaseous or liquid oxidizers. These motors have a combination of features that make solid rocket motors attractive for reasons of simplicity, but like all liquid rocket motors these motors may be extinguished and restarted.
As stated above, rocket motors need to be ignited. Most solid propellant motors use a one use explosive squib. All liquid and hybrid propellant motors often use electric matches, squibs or spark igniters with/or without a combination of a secondary gas flame. The squib systems can only be restarted if the squib is replaced, which is generally not possible for upper stages that are already in flight. External flame sources may be possible on upper stages, but these sources add weight. External flame sources also need to be ignited and have to carry additional spark igniters to ignite these flames. This adds weight and complexity to the system. Hypergolic propellants may overcome this problem, but current hypergolic propellants are toxic and the choice of using hypergolic propellants limits performance and choice of oxidizer and fuel.